In recent years, a new type of system called the identification (ID) recognition camera system (or ID cam) has been proposed. It includes an optical beacon for emitting a blinking signal containing ID information and other items of information relating to electronic devices and a camera with a high-speed image sensor. For example, a system disclosed in Patent Document 1 captures a series of images with an ID-recognition camera and outputs them as a scene image. Then, it decodes the blinking data of the optical beacon by use of all pixel values to create an ID image. Practical applications of this ID-recognition camera system have been also proposed, such as an audio assist system disclosed in Patent Document 2 and an automatic photography system disclosed in Patent Document 3.
The above-described ID-recognition camera system has the function of displaying an image captured with the camera, on which the ID information of each of the optical beacons detected within the captured image is shown at or near the detection point of each optical beacon, allowing users to select some of the ID information according to necessity and to use the related information. Such a system enables users to select one of the electronic devices and indicators and perform data communications with the selected device or control it, using a handheld information terminal.
On the other hand, various techniques using visible light for information transmission and communication have been drawing people's attention. Some of the new approaches are aimed at data communication using indoor or outdoor lighting, traffic signals, indicators of electronic devices or similar light emitters as optical beacons (for example refer to Non-Patent Document 1).
Any system employing such an optical beacon requires a special imaging device for extracting identification information transmitted by the optical beacon present within the captured images, which are normal, two-dimensional images. In general, the signal-reading speed of an image sensor used in a normal imaging device is determined in conformity to the standard frame frequency. Typically, the frame frequency is 30 Hz (frames per second: fps). In contrast, the blinking (or intensity-modulating) frequency of an optical beacon depends on the transmission rate of the identification information. To ensure an adequate amount of information to be transmitted, the blinking or intensity-modulating frequency should be minimally several hundreds Hz, preferably 1 to 100 kHz or higher. Thus, the frequency of the optical beacon is much higher than the frame frequency. Therefore, it is difficult to correctly detect the optical beacon by generally known methods for reading pixel signals from normal image sensors.
Conventionally, some image sensors that can be used in the aforementioned type of system have been proposed. For example, a system disclosed in Non-Patent Document 2 acquires the ID signals by reading pixel signals at a frame rate of 10 kfps, which is much higher than the standard frame frequency. This method is suitable to create a high pixel density device since it allows the use of a pixel circuit approximately identical to those used in normal image sensors without hardly increasing the pixel size. However, this system consumes a considerable amount of electricity; therefore it is necessary to supply a larger amount of current to the circuit components, such as an amplifier for reading the signals, since the operation frequency of the signal-reading circuit increases with the frame frequency. For example, the power consumption of the image sensor disclosed in Non-Patent Document 2 is as high as 2 Watts, which is considerably higher than those of normal image sensors having an equal number of pixels. Incorporating such an image sensor into a small-sized information device is impractical because it will significantly shorten the operating time of the device. Furthermore, use of an image sensor consuming so much power makes the device difficult to design because the heat-releasing performance must be considered. Another problem is that the signal-to-noise (S/N) ratio of the readout signal is low. This is because the readout noise is high due to the high frequency band of the signal-reading circuit while the signal level is low due to the shorter period of time for collecting electric charges.
In a system shown in Non-Patent Document 3, an analogue circuit for detecting the light intensity modulation component of the ID signal transmitted by the optical beacon is provided for each pixel so that the identification information can be extracted within a pixel cell receiving the light emitted from the optical beacon. This method is advantageous in that, even though the signal change is minimal, the modulation component can be detected with a high S/N ratio since each pixel cell has inside a high-gain amplifier. However, this method is not suitable for creating a high pixel density device because the pixel size is considerably large due to the use of more transistors in one pixel circuit than in a pixel circuit of normal image sensors. Moreover, another factor impeding the creation of a high pixel density device is that the power consumption per pixel cell is much higher than in conventional cases since electric current must be continually supplied to the amplifier to operate the analogue circuit inside each pixel cell. This is another factor that impedes the creation of a high pixel density device.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 2003-323239
[Patent Document 2] Unexamined Japanese Patent Application Publication No. 2003-345376
[Patent Document 3] Unexamined Japanese Patent Application Publication No. 2003-348390
[Non-Patent Document 1] “Kashikou Tsuushin Toha (What is Visible Light Communication?”), [Online], Kashikou Tsuushin Consohshiamu (Visible Light Communications Consortium), [Searched on Oct. 15, 2004] Internet <URL: http://www.vlcc.net/about.html>
[Non-Patent Document 2] Miyauchi et al., “Kousoku CMOS Imeeji Sensa Wo Mochiita Nijigen Soujushinki Ni Yoru Heiretsu Hikari Kuukan Tsuushin No Teian (Parallel Optical Wireless Communication using High Speed CMOS Image Sensor”), Shingakugihou (The Technical Report of the Institute of Electronics Information and Communication Engineers of Japan), CS2004-18, 2004
[Non-Patent Document 3] Oike et al., “Fukugou Genjitukan Ouyou Ni Muketa Kousoku-Teikido ID Biikon Kenshutsu Imeeji Sensa (Smart Image Sensor with High-speed High-sensitivity ID Beacon for Augmented Reality System)”, Eijougakkaishi (The Journal of the Institute of Image Information and Television Engineers), Vol. 58, No. 6, pp. 835-841, 2004